Sealing arrangement using flexible seals

ABSTRACT

A sealing arrangement is provided between relatively rotatable bodies of a machine in which, in operation, a relatively high pressure (P 1 ) exists on one side of the sealing arrangement and a relatively low pressure (P 2 ) exists on the other side, the pressure difference (P 1 -P 2 ) being up to a predetermined maximum pressure difference (ΔP max ). The sealing arrangement includes a series of seals, the pressure dropping across each successive seal from the high pressure side of the sealing arrangement to the low pressure side. At least two of the seals are flexible seals, e.g. brush or leaf seals. Each flexible seal  14  is designed to function under a pressure drop (Δp) up to a predetermined maximum pressure drop (Δp max ) across the flexible seal  14  and to provide a given resistance to fluid flow. Each flexible seal  14  is capable of recovering from an abnormal condition, due to normal operation of the machine, in which the resistance of the flexible seal to fluid flow decreases from a normal level to a substantially lower level. The series of seals are arranged in such a manner that the pressure drop across each flexible seal is less than the predetermined maximum pressure drop (Δp max ) when the pressure difference (P 1 -P 2 ) between the high pressure side of the sealing arrangement and the low pressure side is equal to the said predetermined maximum pressure difference (ΔP max ) and another one of the flexible seals has suffered a recoverable decrease in its resistance to fluid flow. In one embodiment the flexible seal  14  is provided with a permanent by-pass path  16  communicating between its upstream and downstream sides.

This application claims priority to British application number 0324076.9, filed 14 Oct. 2003, the entirety of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to sealing arrangements in general, in particular a sealing arrangement between first and second relatively rotatable bodies of a machine, more particularly a turbo machine.

2. Brief Description of the Related Art

The development of flexible seal, e.g. brush seal, or leaf or foil seal, technology over the last two decades has principally been in the field of gas turbines. In such applications, the pressure drop across the seal assembly will not usually be more than 25 bar.

However, in steam turbine applications, sealing arrangements will often have to be designed for pressure drops significantly greater than 25 bar, sometimes more than 50 bar. By way of example, in a high pressure cylinder the steam inlet pressure may be of the order of 200 bar and the exhaust pressure may be of the order of 80 bar. In an intermediate pressure cylinder the inlet pressure may be about 80 bar and the exhaust pressure about 5 bar. Thus, a pressure drop across a balance piston and a pressure drop between the exhaust pressure and the external atmospheric pressure will often be considerably higher than typical applications of flexible seals.

Various seal arrangements using flexible seals such as brush seals have been proposed with the aim of achieving good efficiency and long life. EP-A-0 836 040 discloses a sealing arrangement comprising a series of sealing units 1 (one of which is shown in FIG. 1) between a stationary casing 2 and the rotor 3 of a turbo machine. Each sealing unit 1 includes several brush seals 4 fixed on a carrier 6 mounted on the casing 2 so as to be movable to a limited extent in the radial direction. Fin seals 7 fixed to the rotor 3 act as relatively rigid labyrinth elements.

GB-B-2 301 635 discloses a somewhat similar sealing arrangement (as shown in FIGS. 2 and 3), in which the carrier 6 has a only a single brush seal 4, which is mounted so as to be capable of limited radial movement relative to the carrier 6. The fin seals 7 (labyrinth elements) are provided on the carrier 6.

With flexible (e.g. brush or leaf) seals, non-linear movement of the flexible elements, due to friction between the flexible elements and their supporting structure, can result in situations where one of the seals in a multiple seal configuration is exposed to a significantly larger proportion of the total pressure drop than the design intent and lead to failure of the flexible element. Once a flexible seal has failed, further failures will occur in a cascade fashion due to the increased loading on the remaining seals.

Resistors in series are a simple electrical analogue of a fluid sealing arrangement in which a pressure drop is carried across multiple seals of any type. For incompressible conditions, if all resistances are equal each seal will be exposed to the same fraction of the total pressure drop. For compressible conditions, if the effective leakage area of each seal is the same, the pressure drop (Δp) across individual seals will increase in the downstream direction because a larger pressure drop is required for the same leakage flow as density decreases. This is true regardless of seal type (brush seal, labyrinth seal, other). The pressure distribution is dependent on the relative magnitude of the resistances of the various seals, not their absolute values.

Now consider multiple flexible seals (e.g. brush seals or leaf/foil seals) in series. By way of an example, consider three identical spring-backed sealing glands, each consisting of a brush seal (or leaf/foil seal) mounted in the middle of six conventional labyrinth seal restrictions (fin seals), as shown in FIG. 3. In this example, the clearance between the labyrinth restrictions and the rotor is 1.0 mm and leakage around the back of the glands can be neglected. The pressure difference (P₁-P₂) across the sealing system is 100 bar, say, and in order to simplify the analysis the flow is assumed to be incompressible.

Under normal operation, the design of the brush seals in the example is such that their performance (resistance to fluid flow) is equivalent to that of a conventional single labyrinth restriction with a clearance to the rotor of 0.1 mm. The brush seals are designed to operate satisfactorily under pressure loads of up to 25 bar (predetermined maximum pressure drop (Δp_(max)) or rated pressure drop). The sealing arrangement can be modelled as 18 resistors of magnitude 1 in series with three resistors of magnitude 10 (based on leakage areas). From the electrical analogue, the pressure drop (Δp) across each brush seal will be 100*10/(3*10+18*1)=21 bar.

The worst recoverable leakage (loss of performance or decrease in resistance) of each brush seal will occur if, following some large radial vibration of the rotor, say, the brush seal bristles become temporarily stuck up behind the backing ring, owing to friction. Under these circumstances the brush seal will perform as a single labyrinth restriction with a clearance equal to that between the brush seal backing ring and the rotor, 1.0 mm in this case, say. Suppose now that, as a result of a large rotor vibration event, the bristles of two of the three brush seals become temporarily stuck up behind the backing ring, but those of the third seal are able to quickly recover to the normal position. The system is now operating as 20 labyrinth restrictions of 1.0 mm in series with the equivalent of a single restriction of 0.1 mm. Under these circumstances, the pressure drop across the one ‘unstuck’ brush seal will be 100*10/(10+20*1)=33 bar. So, this seal would suffer permanent damage from over-loading. Once damage has occurred to one seal, the loading on the other brush seals will be increased when their bristles become free again and consequently the likelihood of further bristle pack over-loading is increased. Once damage has started to occur, any such system has the potential to cascade rapidly towards total failure in this manner.

It would therefore be desirable to be able to provide a sealing arrangement including flexible seals which functioned within acceptable design limits during both normal and abnormal operating conditions such as described above, without suffering permanent damage.

SUMMARY OF THE INVENTION

One aspect of the present invention includes providing a sealing arrangement between first and second relatively rotatable bodies of a machine in which, in operation, a relatively high pressure (P₁) exists on one side of the sealing arrangement and a relatively low pressure (P₂) exists on the other side, the pressure difference (P₁-P₂) being up to a predetermined maximum pressure difference (ΔP_(max)), the sealing arrangement comprising a series of seals, the pressure dropping across each successive seal from the high pressure side of the sealing arrangement to the low pressure side, at least two of the seals being flexible seals, each flexible seal being designed to function under a pressure drop (Δp) up to a predetermined maximum pressure drop (Δp_(max)) across the flexible seal and to provide a given resistance to fluid flow, each flexible seal being capable of recovering from an abnormal condition, due to normal operation of the machine, in which the resistance of the flexible seal to fluid flow decreases from a normal level to a substantially lower level, the series of seals being arranged in such a manner that the pressure drop across each flexible seal is less than the predetermined maximum pressure drop (Δp_(max)) when the pressure difference (P₁-P₂) between the high pressure side of the sealing arrangement and the low pressure side is equal to the said predetermined maximum pressure difference (ΔP_(max)) and another one of the flexible seals has suffered a recoverable decrease in its resistance to fluid flow.

The said predetermined maximum pressure difference (ΔP_(max)) may be more than 20 bar, for example.

The said predetermined maximum pressure drop (Δp_(max)) may be in the range from 10 to 30 bar, for example.

In particular, in the above example, if the performance of each individual flexible seal (e.g. brush seal) is degraded (e.g. by shortening the bristles in some circumferential sections of the seal, or by drilling by-pass holes through the seal, allowing leakage around the back of the seal, or by installing the seals with increased radial clearance, or by careful control of the leakage around the back of the spring-backed gland, or if the seal is split into segments leaving gaps between them), limiting the pressure drop caused by the seal, so that the performance of the flexible seal is reduced to being equivalent to a labyrinth (fin seal) restriction with a clearance of 0.15 mm, then under the same scenario as above (bristles of two brush seals stuck behind the backing ring), the maximum pressure drop (Δp_(max)) that the third brush seal would see is now reduced to 25 bar. Thus, in this case, this seal would not be over-loaded and performance of the system would be unchanged when the bristles of the remaining brush seals returned to their normal operating positions.

There will be a penalty in increased leakage flow through the sealing arrangement if the brush seals are protected in this way. Replacing three of 21 labyrinth restrictions with a clearance of 1.0 mm by three brush seals with an equivalent labyrinth clearance of 0.1 mm reduces leakage flow to 44% ((21*1)/(18*1+3*10)) of the leakage through the sealing arrangement with 21 labyrinth restrictions. If the performance of each brush seal is degraded to a labyrinth clearance equivalents of 0.15 mm, the leakage increases to 55% of the pure labyrinth system.

In summary, limiting the pressure drop caused by the flexible seal enables the seal to be protected against over-pressurisation whilst maintaining a leakage performance that is still significantly better than a corresponding all-labyrinth version of the sealing arrangement. Clearly, the trade-off between leakage performance and flexible seal protection depends upon the number of seals and their operating clearances. The values quoted above are specific to the example used by way of explanation. In arrangements with a greater number of seals (balance pistons and end-glands usually have more than three gland rings) the impact on leakage performance of load protection for the flexible seals would typically be less than that seen in the example.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described further, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is an axial cross-section through a known sealing arrangement;

FIG. 2 is an axial cross-section through part of another known sealing arrangement;

FIG. 3 is an axial cross-section through the sealing arrangement partly shown in FIG. 2;

FIG. 4A is a schematic cross-section (taken on line A-A in FIG. 4B) through a first exemplary embodiment of a brush seal for use in a sealing arrangement according to the present invention;

FIG. 4B is a view in the direction of arrow B in FIG. 4A;

FIGS. 5A and 5B are similar to FIGS. 4A and 4B and show a second exemplary embodiment of the brush seal;

FIGS. 6A and 6B are similar to FIGS. 4A and 4B and show a third exemplary embodiment of the brush seal;

FIG. 7 is a cut away view of upstream side of a fourth exemplary embodiment of the brush seal; and

FIG. 8 is a cut away view of the upstream side of a fifth exemplary embodiment of the brush seal.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The brush seal 14 shown in FIGS. 4A and 4B is a form of flexible seal comprising an annular brush 11 (serving as a ring of flexible sealing material) consisting of a closely packed array of bristles, the outer ends of which are trapped between two metallic backing members 12 a, 12 b which are connected by a weld 13 which also secures the bristles 11. The downstream backing member 12 a is longer than the upstream backing member 12 b, so as to support the brush 11 against the pressure on the upstream side of the brush seal. This construction is one of several brush seal constructions already known.

The brush seal 14 is one of a series of seals in a sealing arrangement between first and second relatively rotatable bodies of a turbine. In a steam turbine, for example, the sealing arrangement may be between a stationary casing and a balance piston, which is part of the rotor. Alternatively or additionally, the sealing arrangement may be provided between a stationary casing and a trunnion on the rotor or at any other location between the stationary casing and the rotor where sealing against a large pressure drop is required. The sealing arrangement comprises a plurality of the brush seals as well as fin seals (labyrinth elements); there may, for example, be at least one fin seal upstream of each brush seal.

As can be seen in FIGS. 4A and 4B, the brush seal 14 is provided with a permanent fluid by-pass path (or leakage path) constituted by a plurality of axial bores 16 (shown here equally spaced, by way of example) extending through the backing members 12 a, 12 b from the upstream side to the downstream side of the brush seal. The brush seal is designed to operate satisfactorily under a pressure drop up to a predetermined maximum (Δp_(max), e.g. 25 bar). When the brush seal 14 is used in series with other seals, by-pass bores 16 reduce the pressure drop (Δp) across the brush seal (compared with the situation in which there are no by-pass bores) to such an extent that the pressure drop across the brush seal remains less than the permissible maximum even if, say, two of the other brush seals in the sealing arrangement simultaneously suffer their greatest recoverable decrease in resistance to fluid flow owing to an abnormal condition due to normal operation of the turbine. In such a situation, the brush seals which are in an abnormal condition may, for example, have a resistance to fluid flow which is substantially equal to that of one of the fin seals.

FIGS. 5A and 5B show an embodiment of the brush seal 14 in which a by-pass path is provided by axial grooves 17 in the backing members 12 a, 12 b. The grooves 17 communicate between the upstream side and the downstream side and extend into the downstream backing members 12 a, as shown at 17 a in FIG. 5A.

In FIGS. 6A and 6B the by-pass path is provided by shortening the bristles in equally spaced circumferential sections of the brush 11, to provide apertures 18 at the free end of the brush remote from the backing members 12 a, 12 b.

In FIG. 7, a backing ring is made up of segments 19 which support respective segments 21 of the brush 11 and which are spaced apart by spacers 22 so that there are gaps in the brush 11. The division of the segments 19, 21 is in the direction of the bristle lay angle.

In FIG. 8 the brush seal is divided into segments along radial lines. Extra leakage area, which can be used to protect the brush seal, automatically results from the gap underneath the exposed sections 23 of the backing ring, and additional leakage area is provided by cutting away parts 24 of the exposed sections 23.

Various modifications may be made within the scope of the invention. For example the brush 11 may be replaced by a flexible leaf or foil. The flexible sealing element (e.g. brush, leaf, or foil) may be carried by any convenient supporting element which provides fluid resistance in parallel to the flexible sealing element; the supporting element may be rigid or deformable. The number of brush seals (or leaf or foil seals) may be varied. A balance piston may, for example, be designed with six sealing units (substantially as shown in FIG. 3, but with brush seals as shown in FIGS. 4A and 4B, 5A and 5B, 6A and 6B, 7, or 8) for sealing against a total pressure drop of, for example, 120 bar. Each sealing unit might contain a single brush seal (or leaf or foil seal) capable of accommodating a pressure drop of up to 30 bar. Any of the embodiments described above (or any other means of providing a by-pass flow) could be used to limit the pressure drop across each brush seal to less than 30 bar in a case in which a limited number of the other brush seals suffered a recoverable decrease in resistance to fluid flow, thus eliminating any risk of over-pressurisation of any individual brush seal.

While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. Each of the aforementioned documents is incorporated by reference herein in its entirety. 

1. A sealing arrangement useful between first and second relatively rotatable bodies of a machine in which, in operation, a relatively high pressure (P₁) exists on one side of the sealing arrangement and a relatively low pressure (P₂) exists on the other side, a pressure difference (P₁-P₂) being up to a predetermined maximum pressure difference (ΔP_(max)), the sealing arrangement comprising: a series of seals, the pressure dropping across each successive seal from the high pressure side of the sealing arrangement to the low pressure side when in operation; at least two of the seals comprising flexible seals; each flexible seal being configured and arranged to function under a pressure drop (Δp) up to a predetermined maximum pressure drop (Δp_(max)) across the flexible seal and to provide a given resistance to fluid flow; each flexible seal being configured and arranged to recover from an abnormal condition, due to normal operation of the machine, in which the resistance of the flexible seal to fluid flow decreases from a normal level to a substantially lower level; the series of seals being arranged in such a manner that the pressure drop across each flexible seal when in operation is less than the predetermined maximum pressure drop (Δp_(max)) when the pressure difference (P₁-P₂) between the high pressure side of the sealing arrangement and the low pressure side is equal to the predetermined maximum pressure difference (ΔP_(max)) and another one of the flexible seals has suffered a recoverable decrease in resistance to fluid flow.
 2. A sealing arrangement as claimed in claim 1, in which each flexible seal comprises a permanent fluid by-pass path communicating between the upstream side and the downstream side of the flexible seal and provides fluid conductance in parallel to the fluid resistance of the flexible seal.
 3. A sealing arrangement as claimed in claim 1, wherein each flexible seal comprises a flexible sealing element and a further element providing a fluid resistance in parallel to the fluid resistance of the flexible sealing element.
 4. A sealing arrangement as claimed in claim 3, wherein the further element includes a plurality of by-pass ducts communicating between the upstream side and the downstream side of the flexible seal.
 5. A sealing arrangement as claimed in claim 4, wherein the plurality of by-pass ducts comprise grooves, bores, or both.
 6. A sealing arrangement as claimed in claim 3, in which each flexible sealing element includes a plurality of apertures extending from the upstream side to the downstream side of the flexible seal.
 7. A sealing arrangement as claimed in claim 6, wherein at least some of the plurality of apertures extend to a free end of the flexible sealing element remote from the further element.
 8. A sealing arrangement as claimed in claim 7, wherein each flexible sealing element comprises sub-divided separate segments which are spaced apart to form said plurality of apertures.
 9. A sealing arrangement as claimed in claim 8, wherein each further element comprises segments which support the respective segments of the flexible sealing element and which are spaced apart.
 10. A sealing arrangement as claimed in claim 1, in which the flexible seals comprise brush seals.
 11. A sealing arrangement as claimed in claim 1, in which the flexible seals comprise leaf seals or foil seals.
 12. A sealing arrangement as claimed in claim 1, wherein the series of seals comprises fin seals individually having a lower resistance to fluid flow than the individual flexible seals.
 13. A sealing arrangement as claimed in claim 12, comprising at least one fin seal upstream of each flexible seal.
 14. A sealing arrangement as claimed in claim 1, wherein the series of seals is configured and arranged so that the pressure drop (Δp) across each flexible seal is less than the predetermined maximum pressure drop (Δp_(max)) even when all other flexible seals have suffered their greatest recoverable decrease in resistance to fluid flow.
 15. A sealing arrangement as claimed in claim 1, wherein the predetermined maximum pressure difference (ΔP_(max)) between the high pressure side and the low pressure side is more than 20 bar.
 16. A sealing arrangement as claimed in claim 1, wherein the predetermined maximum pressure drop (Δp_(max)) across each flexible seal is in the range from 10 bar to 30 bar.
 17. A turbine comprising: a sealing arrangement as claimed in claim 1; said first and second relatively rotatable bodies; and wherein the seal arrangement is positioned between said first and second relatively rotatable bodies.
 18. A turbine as claimed in claim 17, wherein the turbine comprises a steam turbine. 